This invention relates to the testing of transformers, and other inductive devices such as reactors, during the course of their service life, to determine whether there is any appreciable slackening of their windings.
The winding conductors of a transformer or reactor, closely coupled to a core of magnetisable material, product an alternating magnetic field when an alternating current flows through the conductors. The interaction of the current and the field produces a force tending to move the conductors in a direction perpendicular to both the current flow path and the field. Since both the direction of current flow and the direction of the field reverse during alternate half-cycles of the alternating supply, the force on the conductors always acts in the same sense, generally tending to separate the conductors from each other and to move them away from the core.
Since the force rises to a peak and falls to zero in each supply half-cycle, it causes vibration of the conductors at twice the supply frequency. The conductors must be mechanically restrained to minimise this vibration.
One of the ageing effects which occur in transformers is shrinkage of the winding insulation. This results in relaxation of the clamping forces applied to the windings, and the windings can therefore move further under the action of the vibrating force. The effect increases progressively with time, and can eventually result in fatigue failure of the insulation and of the winding retainers.
It will be apparent that the forces generated in high-power transformers, particularly at full-load current, can be very large, and it is necessary to determine, from their to time, whether appreciable slackening of the windings has occurred, so that it can be dealt with before failure of the transformer results. However, this testing has required that the transformer be taken out of service and removed from its tank, so that physical measurements can be taken. Clearly, this is a very expensive and time-consuming operation, and the loss of use of the transformer throughout the period of testing can cause considerable problems.
It is an object of the present invention to provide an improved method and apparatus on-site testing of transformers, and other inductors, to determine whether slackening of this windings has occurred.
It is known that when a short circuit occurs in a transformer then the forces generated tend to distort the windings. The paper by W. J. McNutt et al at pp 1955-1969 of IEEE Trans. Power Apparatus and Systems vol. PAS-89, No. 8, 1970 discloses a method of testing the ability of a transformer to withstand short circuits by shorting a winding, injecting a current and analysing the behaviour of the winding by methods including the observation of distortion of the waveform of the injected short circuit current resulting from changes in the winding capacitance. Although of use in predicating the behavior of a winding under the extreme conditions of a short circuit the method described is insufficiently sensitive to be applicable to more general diagnostic purposes.
GB No. 2028504 discloses a diagnostic method in which current is injected into a winding at a frequency chosen to cause vibration and the output from an accelerometer attached to the housing surrounding the winding is monitored. This method, because it relies upon mechanical detection of vibration suffers from poor sensitivity and difficult in obtaining readily reproducible results.
According to one aspect of the invention, a method of monitoring displacement of a conductor in a transformer or inductor winding comprises causing a supply-frequency alternating current of a predetermined amplitude to flow through the conductor; injecting into the conductor a relatively high-frequency alternating signal at a frequency at or near a resonance frequency of stray capacitances and inductances exhibited by the conductor; and monitoring the magnitude or frequency response of the amplitude modulation of the relatively high-frequency signal caused by displacement of the conductor, to determine thereby parameters relating to the displacement of the conductor.
According to another aspect of the invention, apparatus for monitoring displacement of a conductor in a transformer or inductor winding comprises means to inject into the conductor a relatively high-frequency signal at a frequency at or near a resonance frequency of stray capacitances and inductances exhibited by the conductor while the conductor is subjected to a relatively low-frequency alternating current flow at a predetermined current level; and means to monitor the magnitude or frequency response of the amplitude modulation of the relatively high-frequency signal caused by displacement of the conductor to determine parameters relating to the displacement.